1. Field of the Invention
The present invention relates to a method of detecting a longitudinal force of a tire which can detect a longitudinal force (a force in a tire circumferential direction) applied to the tire at a high precision on the basis of a warp angle of the tire, can accurately comprehend a rolling condition of the tire, and can make a contribution to a control of a vehicle control system, and a longitudinal force detecting apparatus used therein.
2. Description of the Related Art
In recent years, in order to secure a stability and a safety of a running motor vehicle, there has been developed various vehicle control systems such as an antilock brake system (ABS), a traction control system (TCS), an electric stability control (ESC) and the like. Further, in order to control these systems, it is necessary to accurately comprehend a rolling condition of the running tire.
For example, in the ABS, it is important to maintain a rolling state having a road surface friction coefficient as large as possible, and the road surface friction coefficient depends on a slip rate of a wheel under a fixed road surface state. Accordingly, the conventional ABS measures a speed of the vehicle and a rotating speed of the wheel, estimates the slip rate on the basis of the measured speed and rotating speed, and controls a lock/unlock of the tire in such a manner that the slip rate is maintained in a range near a position at which the road surface friction coefficient becomes maximum. In this case, on the assumption that a moving speed of the vehicle body of the motor vehicle is set to a vehicle body speed V1, and the speed of the motor vehicle specified by the rotating speed of the wheel (the tire) is set to a wheel speed V2, the slip rate can be expressed by the following expression.{(V1−V2)/V1}×100(%)
Further, the slip rate 100% means a state in which the wheel speed V2 equals to zero, that is, a state in which the wheel (the tire) slips while being completely locked.
However, since the road surface friction coefficient changes hour by hour in correspondence to the road surface condition, a relation between the road surface friction coefficient and the slip rate is not necessarily fixed. Accordingly, in the lock/unlock control of the tire on the basis of the slip rate, there is a risk that a problem the road surface friction coefficient becomes small is generated, even in the case that the slip rate is maintained in a predetermined range.
In order to correspond to the problem mentioned above, in recent years, there has been proposed a structure which measures a longitudinal force (a force in a circumferential direction) applied to the tire, and determining the road surface friction coefficient on the basis of the measured longitudinal force. Further, as a method of measuring the longitudinal force, the structure in Japanese National Publication of International Patent Application No. 10-506346 (corresponding to International Publication No. WO96/10505) has been known.
In the Japanese National Publication of International Patent Application No. 10-506346 (corresponding to International Publication No. WO96/10505), two reference points are provided on one radial line in a side wall surface, and a magnetic body is arranged in each of the reference points. Further, a magnetic sensor is fixed to two positions opposing to the respective reference points. Further, two magnetic sensors measure a relative temporal shift amount at a time of detecting passage of each of the magnetic bodies. Since the temporal shift amount is in proportion to a warp deformation amount of the tire, it is possible to determine the longitudinal force applied to the tire on the basis of the temporal shift amount.